Field of the Invention
The present invention relates to a soundproof body for motor vehicles suitable for insulating noises and a silencer for motor vehicles suitable for performing soundproofing of, for example, noises propagating into motor vehicles and noises within the motor vehicles.
Description of the Related Art
Conventionally as a soundproof body of this type, an ultralight soundproof body described in Japanese Patent No. 3,930,506 has been proposed. The ultralight soundproof body includes a sound absorption layer and a non-air permeable resonant layer. The non-air permeable resonant layer is adhered to the sound absorption layer through an adhesive layer. Thus, the soundproof body absorbs sounds by producing resonance at an interface between the sound absorption layer and the non-air permeable resonant layer.
By the way, in the soundproof body constructed as described above, the sound absorption layer is an air permeable layer made of felt or the like. On the other hand, there are no changes in the case that the non-air permeable resonant layer is a non-air permeable layer.
Accordingly, when the soundproof body is disposed at the sound absorption layer along a dash panel which is a component to be disposed on a boundary between an engine room and a vehicle compartment of a motor vehicle, engine sounds generated in the engine room propagates as noises, through the dash panel and the soundproof body in this order, and then reaches the interior of the motor vehicle compartment.
Herein, the noises are incident on the dash panel, the sound absorption layer, the adhesive layer, and the non-air permeable resonant layer in this order. Since the sound absorption layer is made of an air permeable material such as felt or the like, the sound absorption layer functions as an air layer.
Thus, in a process in which the noises are incident on the dash panel, the air layer equivalent to the sound absorption layer, the adhesive layer, and the non-air permeable resonant layer in this order, the air layer equivalent to the sound absorption layer functions as a spring layer in between the non-air permeable resonant layer in accordance with the sound pressure of the noises propagating through the dash panel.
As a result, the dash panel and the non-air permeable resonant layer cause a transmission resonance phenomenon under the spring action of the air layer equivalent to the sound absorption layer, thereby to degrade the transmission resonance phenomenon as the sound absorption performance of the soundproof body in a low-frequency range of noises.